Lithium silicate glass ceramics are characterized by very good mechanical properties, which is why they have been used for a long time in the dental field and primarily for preparing dental crowns and small bridges. The known lithium silicate glass ceramics usually contain as main components SiO2, Li2O, Al2O3, alkali metals such as Na2O or K2O and nucleating agents such as P2O5. In addition, they can contain as further components for example further alkali metal oxides and/or alkaline earth metal oxides and/or ZnO. Glass ceramics are also known which contain small quantities of further metal oxides and in particular colouring and fluorescent metal oxides.
EP 1 505 041 and U.S. Pat. No. 7,316,740, which is hereby incorporated by reference in its entirety, describe lithium silicate glass ceramics which can additionally contain 0 to 2 wt.-% ZrO2 as well as 0.5 to 7.5 wt.-% and in particular 0.5 to 3.5 wt.-% colouring and fluorescent metal oxides. EP 1 688 398 and U.S. Pat. No. 7,452,836, which is hereby incorporated by reference in its entirety, describe similar lithium silicate glass ceramics which are substantially free of ZnO and can also contain, in addition to the above-mentioned quantities of colouring and fluorescent metal oxides, 0 to 4 wt.-% ZrO2, wherein however to achieve high strengths smaller quantities of from 0 to 2 wt.-% ZrO2 are preferred. The glass ceramics are processed into the desired dental restorations in particular in the form of lithium metasilicate glass ceramics by means of CAD/CAM methods, wherein a subsequent heat treatment effects the conversion of the metasilicate phase to the high-strength disilicate phase.
U.S. Pat. No. 6,455,451, which is hereby incorporated by reference in its entirety, relates to lithium disilicate glass ceramics which, in addition to other components, can also contain transition metal oxides. It is proposed inter alia, in order to increase the refractive index of the glass matrix, to add small quantities of heavy elements such as Sr, Y, Nb, Cs, Ba, Ta, Ce, Eu or Tb. Thus, for example, CeO2 and Tb4O7 can be used in quantities of from 0 to 1 wt.-%, Nb2O3 and Ta2O5 in quantities of from 0 to 2 wt.-% and ZrO2 and Y2O3 in quantities of from 0 to 3 wt.-%. In one embodiment, Ta2O5 is said to be able to be present in a quantity of from 0.5 to 8 wt.-%, even though the specific examples contain at most 2.02 wt.-% of this oxide.
U.S. Pat. No. 5,176,961 and U.S. Pat. No. 5,219,799, which are hereby incorporated by reference in their entirety, disclose glass ceramics for example for the production of crockery, which can contain as colorants specific transition metal oxides such as CeO2, CO3O4, Cr2O3, CuO, Fe2O3, MnO2, NiO and V2O5 in a quantity of from 0.01 to 7 wt.-%.
U.S. Pat. No. 5,507,981 and U.S. Pat. No. 5,702,514 which are hereby incorporated by reference in their entirety, describe processes for shaping dental restorations and glass ceramics that can be used in these processes. These are in particular lithium disilicate glass ceramics which can contain 0 to 5 wt.-% colouring oxides such as SnO2, MnO, CeO, Fe2O3, Ni2O, V2O3, Cr2O3 or TiO2.
Known glass ceramics based on lithium silicate often have optical properties which do not adequately satisfy the aesthetic requirements in particular in connection with the use as dental materials. Thus known glass ceramics often have an unfavourable refractive index. With glass ceramics in particular there is the problem that the refractive indices of the crystalline phase and of the glass phase usually differ markedly from each other, which in most cases results in an undesired clouding of the glass ceramic. Similar problems exist for example in the case of composites because the refractive indices of known glass ceramics and glasses usually differ from those of the polymer phase. There is therefore a need for glass ceramics based on lithium silicate the refractive index of which can be easily varied, but without the other properties being substantially impaired. Moreover, it is desirable that such glass ceramics can be prepared and crystallized under conditions comparable to those for customary glass ceramics.